Liquid crystal-based electrooptical device forming, in particular, a switch

ABSTRACT

The present invention provides an electro-optical device, characterized in that it comprises two plane optical substrates ( 100 ) each having at least one optical waveguide ( 110 ), and a nematic liquid crystal ( 200 ) inserted between them, in which the liquid crystal ( 200 ) is split into two separate active zones ( 210, 220 ) serving to control coupling/decoupling of a respective one of the TE and TM polarizations of a light signal injected into the waveguides ( 110 ).

[0001] The present invention relates to the field of switching opticalsignals.

[0002] More precisely, the present invention preferably applies toswitching wavelength division multiplexed (WDM) signals propagatingbetween optical ports, e.g. in optical fibers.

[0003] The present invention seeks to design an N×P optical switch whereN and P are integers, and where N and P may optionally be equal.

[0004] The appearance of WDM signals several years ago in opticalcommunications has required new optical components to be developed toenable broad spectrum bands to be processed.

[0005] One of the most critical components is an optical switch enablingoptical signals coming from a plurality of inlet ports or fibers to berouted to different outlet ports or fibers.

[0006] In the past, such routing was performed via opto-electronicconverters. Unfortunately, electronic conversion can process only asingle wavelength and is therefore incompatible with WDM transmission(80 propagated wavelengths at present, 160 in the near future).

[0007] This has led to numerous suppliers specialized in the field oftelecommunications to develop all-optical switches, i.e. switches inwhich the switching process is entirely optical.

[0008] Amongst such all-optical switches, micro-electrical mechanicalsystems (MEMS) have undoubtedly attracted the most enthusiasm in thetelecommunications community ([1] MEMS based photonic switching incommunication networks, by Dr. Anis Husain, OFC 2001 Proceedings, PaperWX1-1). They operate on the principle of activating silicon micromirrorsand they can be used to build switching matrices of any size (from 1×2ports to 4000×4000 ports). Although their performance in terms ofcoupling, insertion losses, switching time, and cross-talk are verygood, MEMS suffer from a high degree of technological complexityassociated with making silicon micromirrors and with the large number ofsuch mirrors in matrices having a large number of ports.

[0009] Other optical switching technologies have been developed inparallel, such as thermo-optical switching, ink bubble switches, orliquid crystal switches.

[0010] The two first-mentioned of those technologies use integratedoptical solutions: thermo-optical switches make use of thethermo-optical effect generated in a Mach-Zehnder interferometer builtusing planar optics, while ink bubble switches use the total reflectioncaused by locally heating bubbles of ink placed at the intersectionsbetween a plurality of plane optical waveguides ([2] U.S. Pat. No.6,212,308). Although those technologies present the advantage of usingsolutions that are optically fully integrated, they generally do notpresent good stability with respect to temperature or time.

[0011] Presently developed liquid crystal technologies use thepolarization rotation that is induced by propagating optical signalsthrough a liquid crystal cell ([3] U.S. Pat. No. 6,134,358).Unfortunately, the operation of those switches requires differentpolarizations to be processed separately, thereby complicating theoverall optical design of such a component. A second drawback is due tothe fact that constructing an N×P matrix having a large number of portscan be envisaged only by using a plurality of liquid crystal cells incascade, which implies a great increase in insertion losses withincreasing number of matrix ports.

[0012] An object of the present invention is to propose novel opticalswitching means making it possible in particular to overcome thedrawbacks associated with separate processing for differentpolarizations.

[0013] An auxiliary object of the present invention is to propose meansenabling an optical signal to be attenuated progressively in controlledmanner between at least one inlet port and one outlet port.

[0014] In the context of the present invention, this object is achievedby an electro-optical device comprising two plane optical substrateseach having at least one optical waveguide, and a nematic liquid crystalinserted between them, in which the liquid crystal is split into twoseparate active zones serving to control coupling and decoupling of arespective one of the TE and TM polarizations of a light signal injectedinto the waveguides.

[0015] According to an advantageous characteristic of the presentinvention, the device has two pairs of electrodes associated withrespective ones of the two active liquid crystal zones, the electrodesin each pair being disposed on respective opposite sides of thewaveguide, and the orientations of the electrodes being mutuallyorthogonal from one pair to the other.

[0016] Other characteristics, objects, and advantages of the presentinvention appear on reading the following detailed description withreference to the accompanying drawings, given as non-limiting examples,and in which:

[0017]FIG. 1 is a diagrammatic perspective view showing the basicstructure of a device in accordance with the present invention;

[0018]FIG. 2 is a similar view of the device, in its position whenactivated by applying an electric field;

[0019]FIG. 3 is a plan view of the substrate and shows the positions ofthe active liquid crystal and optical signal zones;

[0020]FIG. 4 is a diagrammatic perspective view of a switch inaccordance with the present invention;

[0021]FIG. 5 is a cross-section view of the switch on the section planereferenced V-V in FIG. 4, the device being shown in its activatedposition;

[0022]FIG. 6 is a longitudinal section view of the switch, in the reststate;

[0023]FIG. 7 is a similar longitudinal section view of the switch in theactivated state;

[0024]FIGS. 8 and 9 are two cross-section views respectively on sectionplanes referenced VIII-VIII and IX-IX in FIG. 1 showing a preferredembodiment, and thus showing a specific way of implanting electrodes;

[0025]FIG. 10 is a detailed section view of the device revealing abuffer layer (coherent length) of liquid crystal at the interfaces;

[0026]FIG. 11 is a diagrammatic section view of the device in a controlstate suitable for imparting an optical attenuation effect; and

[0027]FIGS. 12, 13, and 14 are diagrams showing three possibleembodiments of switches in accordance with the present invention.

[0028] The device of the present invention is made using integratedoptics and it makes use of the ability of a liquid crystal placedbetween two substrates to be reoriented electrically.

[0029] The switch of the present invention is made using integratedoptics and it makes use of the ability of a liquid crystal placedbetween two plane substrates to be reoriented electrically.

[0030] In the context of the present invention, the inlet and outletports are preferably embodied by optical fibers. Nevertheless, in avariant, the inlet ports may be formed by any equivalent light emitter;similarly, the outlet ports may be formed by any equivalent lightreceiver.

[0031] The basic structure of the electro-optical device in accordancewith the present invention as shown in FIGS. 1 to 3 is describedinitially.

[0032] This basic structure comprises:

[0033] a plane substrate 100 possessing an optical waveguide 110;

[0034] a nematic liquid crystal material 200 placed against thesubstrate 100 in the form of two separate active zones 210 and 220; and

[0035] at least one pair of electrodes 310 & 312 and 320 & 322associated with each active zone 210, 220 of the liquid crystal 200 andplaced respectively on either side of each of these zones in anorientation suitable for processing TE and TM linear polarization,respectively.

[0036] The description below is given with reference to an x, y, zorthogonal frame of reference having origin O, in which the axis Oxextends perpendicularly to the mean plane of the substrate 100, the axisOy extends parallel to the substrate 100 and perpendicularly to thelongitudinal direction of the optical waveguide 110, and the axis Ozextends parallel to the substrate 100 and parallel to the opticalwaveguide 110.

[0037] As shown in accompanying FIG. 1, the waveguide 110 is rectilinearalong the axis Oz. It is flush with one of the main surfaces 102 of thesubstrate 100. The waveguide 110 defines an inlet port 112 at one of itsends and an outlet port 114 at its other end (this definition of aninlet port 112 and an outlet port 114 is nevertheless arbitrary insofaras the device is symmetrical and consequently each of the ports 112 and114 can equally well be an inlet port or an outlet port).

[0038] The optical waveguide 110 implanted in the plane substrate 100 ismade in such a manner as to be capable of conveying only the twofundamental modes TE₀ and TM₀. These modes are polarized in respectivedirections Oy and Ox, as shown in FIG. 1.

[0039] Thus, the waveguide 110 is preferably of quadrangular rightsection, being square or rectangular, with facets that are respectivelyparallel and perpendicular to the axes Oy and Oz and to the main facesof the substrate 100.

[0040] The substrate 100 and the waveguide 110 are advantageously madeusing silica or a polymer material.

[0041] The nematic liquid crystal 200 possesses an ordinary refractiveindex no that is less than the refractive index n_(g) of the opticalwaveguide 110, and its possesses an extraordinary refractive index n_(e)that is greater than n_(g).

[0042] The anchoring of the liquid crystal 200 on the plate 100 and alsoon the facing interface plate (not shown in FIG. 1 in order to simplifythe figure) must be weak so as to minimize the disturbance from thebuffer layer of the liquid crystal at the interfaces when an electricfield is applied that is high but less than the breakage electric field,or so as to diminish the value of the electric field that is appliedwhen anchoring is broken, with this taking place when an appropriatevoltage is applied across the electrode pairs 310 & 312 or 320 & 322.

[0043] The two active zones 210 and 220 of nematic liquid crystal areplaced facing the waveguide 110. They are spaced apart in the Ozdirection by a non-active zone 230. As can be seen in accompanying FIGS.1 to 3, the width of the active zones 210 and 220 in the Oy direction isgreater than the corresponding width of the waveguide 110.

[0044] Typically, each active zone 210 and 220 of liquid crystal is ofquadrangular right section in the zOy plane.

[0045] The two electrodes 310 & 312 associated with the zone 210 aredisposed respectively on either side of the liquid crystal in the Oxdirection. In the absence of an electrical voltage across the electrodes310 & 312, the molecules of the liquid crystal 210 are oriented parallelto the waveguide 110 in the Oz direction, as shown diagrammatically inFIG. 1. In contrast, when an appropriate voltage is applied across theelectrodes 310 & 312, the molecules of the liquid crystal 210 situatedbetween those electrodes become oriented perpendicularly to the longdirection of the waveguide 110 in the Ox direction, as showndiagrammatically in FIG. 2.

[0046] Thus, applying a voltage across the electrodes 310 & 312 servesto decouple the TM-polarized component of the signal injected into thewaveguide 110.

[0047] In practice, the two electrodes 310 and 312 can be supportedrespectively on the outside surface of the plate 100 and on the outsidesurface of the facing confinement plate.

[0048] The two electrodes 320 & 322 associated with the zone 220 aredisposed respectively on either side of the liquid crystal in the Oydirection. In the absence of an electric voltage across these electrodes320 & 322, the molecules of the liquid crystal 220 are oriented parallelto the waveguide in the Oz direction, as shown diagrammatically inFIG. 1. In contrast, when an appropriate voltage is applied across theelectrodes 320 & 322, the molecules of the liquid crystal 220 situatedbetween these electrodes become oriented perpendicularly to the longdirection of the waveguide 110 in the Oy direction, as showndiagrammatically in FIG. 2.

[0049] Thus, applying a voltage across the electrodes 320 & 322 servesto decouple the TE-polarized component of the signal injected in thewaveguide 110.

[0050] In practice, the two electrodes 320 and 322 can be supporteddirectly by the plate 100.

[0051] Each active zone 210, 220 of liquid crystal thus serves to coupleor decouple either the TM₀ mode or the TE₀ mode from the inlet port 112going towards the outlet port 114 situated at the outlet of the opticalwaveguide 110, or vice versa.

[0052] In the absence of any applied voltage between the electrodes 310& 312 or 320 & 322, the light signal injected via one of the ports 112or 114 arrives in full at the port 114 or 112 situated at the other endof the waveguide 110. In contrast, in the presence of an appropriateelectric field, the signal applied to the inlet of the waveguide 110 isdecoupled, and consequently does not appear at the outlet.

[0053] By way of non-limiting example:

[0054] insertion losses for a 2×2 coupler are of the order of 0.5decibels (dB) to 1 dB;

[0055] the width of the waveguide 110 in the Oy direction is about 4micrometers (μm) to 8 μm;

[0056] the thickness of the waveguide 110 in the Ox direction is about 2μm to 4 μm;

[0057] the thickness of the liquid crystal 200 between the plate 100 andthe facing confinement plate is about 2 μm to 6 μm;

[0058] the length of each active zone 210, 220 of liquid crystalmeasured in the Oz direction is about 50 μm to 100 μm;

[0059] each active zone 210, 220 of liquid crystal is of a width in theOy direction that is greater than the width of the waveguide 110,typically lying in the range 10 μm to 30 μm; and

[0060] the electric field applied across the electrodes 310 & 312 or 320& 322 is of the order of 3 volts per micrometer (V/μm) to 10 V/μm.

[0061] There follows a description with reference to FIGS. 4 to 7 of thebasic structure of a light switch in accordance with the presentinvention having a 2×2 configuration, i.e. possessing two inlets and twooutlets, the signal present on each of the two inlets being capable ofbeing applied alternately to a selected one of the two outlets.

[0062] Naturally such a device could be used as a 1×2 switch if only oneinlet of the device is used, with the structure of the device otherwiseremaining identical to the means described below.

[0063] The 2×2 matrix is fabricated using two plane substrates 100, 400that are symmetrical about the yOz plane. Each substrate 100, 400possesses a respective implanted plane optical waveguide 110, 410. Thetwo waveguides 110, 410 are comparable to the waveguide 110 describedabove. The two plane substrates 100, 400 are placed one above the other,with the two waveguides 110, 410 being parallel and superposed so as toconstitute a vertical coupler. The two waveguides 110, 410 are thusseparated by a liquid crystal medium 200.

[0064] This device also has the liquid crystal 200 split into two activezones 210, 220 that are spaced apart in the Oz direction along thewaveguides 110 and 410, and it has the two pairs of electrodes 310 & 312and 320 & 322.

[0065] The electrodes 310 & 312 spaced apart in the Ox direction anddisposed respectively on opposite sides of the zone 210 can be carriedby the outside surfaces of the plates 100 and 400 respectively, as canbe seen in FIG. 4 for the electrode 310.

[0066] The electrodes 320 & 322 spaced apart in the Oy direction andplaced on either side of the zone 220 are advantageously carried by theinside surfaces of the plates 100 and 400.

[0067] In practice, the thickness of the electrodes 320 & 322 can beequal to the width of the gap between the two plates 100 and 400, oreach electrode can be subdivided into a group of electrodes of thicknesssmaller than said gap, respective smaller-thickness electrodes beingadjacent to each of the plates 100, 400 and separated by a spacer (asshown in FIG. 8).

[0068] Naturally, under such circumstances, the control voltage (+v; −v)is applied both across the pair of electrodes 320 a & 322 a carried bythe plate 100 and across the pair of electrodes 320 b & 322 b carried bythe plate 400, so as to define electric fields oriented along the Oydirection.

[0069] Similarly, the electrodes 310 & 312 can be placed on the insidefaces of the plates 100 and 400, and each electrode can be split into agroup of electrodes of thickness smaller than the gap between the twoplates 100, 400, the smaller-thickness electrodes being respectivelyadjacent to each of the plates 100, 400 and being carried thereby in themanner shown in FIG. 9. The control voltage (+v; −v) is then appliedacross the electrode pair 312 a & 310 a carried respectively by theplates 100 and 400 on one side of the zone 210, and by the electrodepair 312 b & 310 b carried respectively by the plates 100 and 400 on theother side of the zone 210, thereby defining electric fields orientedalong the Ox direction.

[0070] Such electrodes 310 a, 310 b, 312 a, 312 b, 320 a, 320 b, 322 a,and 322 b are typically made of aluminum having thickness of about 100nanometers (nm) to 500 nm. They are formed outside the waveguides 110,410 so as to avoid absorbing the light signal.

[0071] The two inlet signals are injected into the two waveguides 110,410 inserted in the plane substrates 100, 400, e.g. via inlet portsreferenced 112 and 412 in the accompanying figures.

[0072] By selecting a liquid crystal 200 whose ordinary index n_(o) isless than the index n_(g) of the optical waveguides 110, 410 and whoseextraordinary index n_(e) is greater than n_(g), it is possible tocouple one of the linear TE or TM polarizations from the ports 112 and412 to the outlet ports at the opposite ends 114 and 414 by using anactive liquid crystal zone.

[0073] Two states (active and not active) are associated with each ofthe active liquid crystal zones 210 and 220, depending on whether or notan external electric field is applied to the liquid crystal zone inquestion by means of the electrodes 310 & 312, 320 & 322.

[0074] In the absence of an electric field (non-active state), theanchoring of the liquid crystals to the interfaces with the substrates100, 400 determines the orientation of the liquid crystal in the volume.The liquid crystal molecules are thus oriented parallel to the Ozdirection and to the longitudinal direction of the waveguides 110, 410,as can be seen in FIG. 6.

[0075] In the presence of an electric field (active state), theorientation of the liquid crystal in the volume is determined by thedirection of the electric field applied across the substrates 310 & 312or 320 & 322.

[0076] In the presence of such a field, in the zone 210 between thesubstrates 310 & 312, the liquid crystal becomes oriented in the Oxdirection as shown in FIGS. 5 and 7, while in the zone 220 between thesubstrates 320 & 322, the liquid crystal becomes oriented along the Oydirection, as shown in FIG. 7.

[0077] In order to ensure that the light signals injected to the inlets112 and 412 are transferred in full to the outlets 114 and 414, it isessential to couple both the TE and the TM polarizations. In order to dothis, in the context of the invention, two active liquid crystal zones210, 220 are used which are spaced apart by a non-active distance 230.The alignments selected for the liquid crystal and the directions of theapplied electric fields are determined so as to define two liquidcrystal zones that process the TE and the TM polarizations insuccession.

[0078] Practical implementation of the two active zones 210, 220 ofliquid crystal requires that the electrodes 310 & 312, 320 & 322 beinstalled so as to reorient electrically the nematic liquid crystal ineach of these zones. The electrodes 320 & 322 are implanted on eitherside on the plane substrates 100, 400 for the active zone 220 processingTE polarization on either side of each of the optical waveguides 110,410 (e.g. as shown in FIG. 8). This configuration for the electrodesmakes it possible to obtain an electric field in the Oy direction andthus to reorient the liquid crystal in said direction. For the activezone 210 processing the TM polarization, the electrodes 310 & 312 areimplanted beneath the optical waveguides 110, 410 so as to produce anelectric field in the Ox direction, or else they are implanted on eitherside of the waveguides 110, 410, as shown in FIG. 9.

[0079] The liquid crystal 200 can be confined in at least two ways. Thefirst way consists in filling the entire gap between the two planesubstrates 100, 400 with liquid crystal 200. Under such circumstances,counter-electrodes define the boundaries between the active zones 210,220 and the non-active zone 230 of the liquid crystal. The second wayconsists in using a medium of index that is lower than that of theoptical waveguides 110, 410, said medium defining the boundaries of theactive liquid crystal zones 210, 220.

[0080] In order to optimize coupling and minimize losses in the 2×2switch, the plane optical waveguides 110, 410 made in the planesubstrates 100, 400 are capable of propagating only the fundamental TE₀and TM₀ modes. Thus, any incident polarization entering the ports 112and 412 of the switch can propagate without loss in the switch. Sincethe TE₀ and TM₀ modes are normal modes, coupling one of these modes inone of the active liquid crystal zone 210, 220 has no influence on theother propagated mode. It is thus entirely possible to couple the TE₀mode without disturbing propagation of the TM₀ mode in an active liquidcrystal zone, and vice versa.

[0081] The 2×2 switch in accordance with the present invention operatesentirely symmetrically relative to the inlet ports 112, 412 and theoutlet ports 114, 414. This means that the light signals injectedrespectively to the inlets 112 and 412 cannot be switched to the sameoutlet port. Thus, for example, if the inlet signal injected into theinlet 112 is directed towards the outlet port 114, then the inlet signalinjected to the inlet 412 is necessarily directed to the outlet port414.

[0082] In order to minimize residual cross-over losses in the 2×2 switch(residual cross-talk losses), it is essential to minimize thedepolarization of the TE₀ and TM₀ modes while the light signals passthrough the two active liquid crystal zones 210, 220. Any depolarizationconstitutes loss by coupling to the wrong outlet port in a device of thetype in accordance with the present invention that has two active zonesin series. It is consequently essential to select a configuration forthe active liquid crystal zones 210, 220 (i.e. alignment directions andreorientation directions of the liquid crystal) that is compatible withthe directions specific to the TE and TM polarizations. Secondly, it isnecessary to select an appropriate value for the liquid crystalanchoring energy. There generally exists a buffer layer in which theliquid crystal becomes progressively reoriented under the application ofan external electric field. Such a buffer layer is showndiagrammatically in FIG. 10. The thickness of this buffer layer(coherent length) and its optical characteristics depend essentially onthe anchoring force between the liquid crystal and the plane substrates.The most effective way of minimizing its influence on the coupling andthus of minimizing depolarization in the switch of the inventionconsists in using an anchoring force that is weak both in the zenithdirection and in the azimuth direction relative to the plane substrates100 and 400. This weak anchoring energy is selected so as to conserve anacceptable response time for the liquid crystal (t<20 milliseconds(ms)).

[0083] The structure described above is suitable for providing on/offswitching of light signals between inlet ports and outlet ports byswitching the orientation of liquid crystal molecules between a restorientation in the Oz direction and orientations under applied voltagein the Oy and the Ox directions respectively.

[0084] Nevertheless, by applying voltage across the electrodes 310 & 312and 320 & 322 that is less than the voltage required for achieving suchon/off switching, the molecules are merely inclined away from the Ozdirection respectively towards the Oy direction or the Ox direction.Under such circumstances, the inlet signals are not transferred in fullto the outlets but they are merely attenuated.

[0085] The amplitude of the voltage applied across the substrates 310 &312 and 320 & 322 can thus be used to control the attenuation of thelight signals over the range 0 to 100%.

[0086]FIG. 11 is a diagram showing an intermediate configuration for theliquid crystal obtained with a voltage of amplitude controlled in thisway so as to enable an attenuation effect to be obtained.

[0087] The present invention is not limited to making a switch orattenuation matrix having a 2×2 configuration. It extends to any otherconfiguration of the N×N type or more generally of the N×P type.

[0088] Examples of N×N or N×P switches are shown in FIGS. 12 to 14.

[0089] An N×N or N×P switch matrix having a large number of ports isbuilt up in a manner similar to building 2×2 matrices.

[0090] Such N×N switches comprise two plane substrates 100, 400 with aplurality of waveguides 110, 410 identical to those used in the 2×2switches. Certain segments of the waveguides 110, 410 provided on thetwo substrates 100, 400 are placed facing one another. These segmentsare separated by pairs of active liquid crystal zones 210 & 220 eachzone being associated with a pair of electrodes 310 & 312 and 320 & 322.

[0091] The junction between the various waveguides in theabove-mentioned segments is preferably implemented by using curvedwaveguides that carry only the fundamental TE₀ and TM₀ modes.

[0092] According to a preferred characteristic of the invention, theswitches made in this way advantageously comprise N(N−1)/2 individual2×2 switches that are interconnected.

[0093]FIGS. 12 and 13 show in non-limiting manner two examples of N×Nswitches. In these figures, one of the substrates 100 and the associatedwaveguides 110 are represented by continuous lines. In contrast, theother substrate 400 and its associated waveguide 410 are sketched asdashed lines.

[0094]FIG. 12 shows an example of a 4×4 switch (i.e. a switch havingfour inlet ports and four outlet ports) in which six switch zones 200are provided.

[0095]FIG. 13 shows an example of a 6×6 switch (i.e. six inlet ports andsix outlet ports) in which 15 switch zones 200 are provided.

[0096] The illustrations and configurations shown in FIGS. 12 and 13 arenot limiting.

[0097] In these figures, the N waveguides 110, 410 are provided inalternation on the two substrates 100, 400. For waveguides of rank 1 toN-1, the waveguide of rank i is coupled via i-1 switches to a firstwaveguide of the other substrate and via i switches to a secondwaveguide of said other substrate.

[0098] A variant embodiment of the switch as shown in accompanying FIG.14 is described below.

[0099] The example given in FIG. 14 corresponds to a 3×3 switch. Thestructure shown in FIG. 14 is nevertheless easily generalizable to anN×N switch and more generally to an N×P switch.

[0100] In FIG. 14, one of the substrates 100 and the associatedwaveguides 110.1, 110.2, and 110.3 are drawn in continuous lines whilethe other substrate 400 and its associated waveguides 410.1, 410.2, and410.3 are drawn as dashed lines.

[0101] Each waveguide 110, 410 has a plurality of segments that are notin alignment such that each waveguide 110 provided on the substrate 100presents a segment facing each waveguide 410 provided on the secondsubstrate 400.

[0102] For each assembly E comprising such a pair of facing segments oftwo waveguides 110, 410 belonging to the two different substrates 100,400, there is to be found the same basic switch structure as describedabove comprising two separate active zones 210, 220 and two orthogonalpairs of electrodes 310 & 312 and 320 & 322.

[0103] For an N×P switch, there are thus NP such assemblies E. Each ofthese NP assemblies E can be controlled individually and separately fromthe other sets.

[0104] More precisely, the waveguides 110, 410 situated respectively onthe two substrates 100 and 400 preferably extend in directions that aregenerally mutually orthogonal.

[0105] For example, the waveguides 110 of the substrate 100 extendbetween two mutually parallel edges 104 and 106 of the substrate.

[0106] The waveguides 410 of the substrate 400 extend between twomutually parallel edges 405 and 407 of the substrate 400, which edgesare orthogonal to the above-mentioned edges 104, 106.

[0107] In addition, the waveguides 110, 410 are in a staircaseconfiguration.

[0108] Thus, the waveguides 110 of the substrate 100 have rectilinearsegments 111 that are orthogonal to the above-mentioned edges 104, 106and that are interconnected by rectilinear segments 113 that areinclined relative to said edges 104, 106, and that are preferably at 45°relative thereto.

[0109] Similarly, the waveguides 410 of the substrate 400 compriserectilinear segments 411 that are orthogonal to the above-mentionededges 405, 407 and that are interconnected to one another by rectilinearsegments 413 that are inclined relative to said edges 405, 407,preferably at 45° relative thereto, so that said segments 113 and 413are parallel to one another and face one another.

[0110] The structure thus formed and shown in FIG. 14 enables the signalto be transferred from any one of the waveguides 110 to any one of thewaveguides 410 (and vice versa) by appropriately controlling and passingthrough only one of the switch assemblies E.

[0111] Thus, in order to transfer a signal applied to the inlet ofwaveguide 110.1, to one of the waveguides 410.1, 410.2, or 410.3, it isappropriate to operate the corresponding switch assembly E₁₁, E₁₂, orE₁₃.

[0112] In order to transfer a signal applied to the inlet of waveguide110.2 to one of the waveguides 410.1, 410.2, or 410.3, it is appropriateto operate the corresponding one of the switch assemblies E₂₁, E₂₂, orE₂₃.

[0113] More generally, in order to transfer a signal applied to theinlet of waveguide 110.i to a waveguide 410.j, it is necessary tooperate the switch assembly E_(ij).

[0114] Naturally, the present invention is not limited to the particularembodiments described above, but it extends to all variant embodimentscoming within the spirit of the invention.

[0115] The above description relates to liquid crystals having positivedielectric anisotropy, in which the molecules of the liquid crystalorient themselves parallel to the applied electric field.

[0116] The person skilled in the art can easily adapt the orientationsof the electrodes 310 & 312 and 320 & 322 for use with liquid crystalshaving negative dielectric anisotropy, in order to obtain the desiredattenuation or switching, it being recalled that under suchcircumstances, the liquid crystal molecules orient themselvesperpendicularly to the applied electric field.

1/ An electro-optical device, characterized in that it comprises twoplane optical substrates (100, 400) each having at least one opticalwaveguide (110, 410), and a nematic liquid crystal (200) insertedbetween them, in which the liquid crystal (200) is split into twoseparate active zones (210, 220) serving to control coupling anddecoupling of a respective one of the TE and TM polarizations of a lightsignal injected into the waveguides (110, 410). 2/ A device according toclaim 1, characterized by the fact that it has two pairs of electrodes(310 & 312; 320 & 322) associated with respective ones of the two activeliquid crystal zones (210, 220), the electrodes (310 & 312; 320 & 322)in each pair being disposed on respective opposite sides of thewaveguide (110, 410), and the orientations of the electrodes (310 & 312;320 & 322) being mutually orthogonal from one pair to the other. 3/ Adevice according to claim 1 or claim 2, characterized by the fact thatthe waveguide (110) is rectilinear and flush with one of the mainsurfaces (102) of the substrate (100). 4/ A device according to any oneof claims 1 to 3, characterized by the fact that the waveguide (110) isof quadrangular right section, square or rectangular, with facets thatare respectively parallel and perpendicular to the main faces of thesubstrate (100), in such a manner as to allow only the fundamental TE₀and TM₀ modes to propagate. 5/ A device according to any one of claims 1to 4, characterized by the fact that the nematic liquid crystal (200)possesses an ordinary index n_(o) that is lower than the index n_(g) ofthe optical waveguide (110), and an extraordinary index n_(e) that isgreater than the index n_(g) of the optical waveguide (110). 6/ A deviceaccording to any one of claims 1 to 5, characterized by the fact thatthe anchoring of the liquid crystal (200) on the plate (100) is weak. 7/A device according to any one of claims 1 to 6, characterized by thefact that the liquid crystal (200) is confined by means ofcounter-electrodes defining the boundary between the active andnon-active zones (210, 220; 230) of liquid crystal. 8/ A deviceaccording to any one of claims 1 to 6, characterized by the fact thatthe liquid crystal (200) is confined by means of a medium of index lowerthan that of the optical waveguide (110, 410), which medium defines theboundaries of the active liquid crystal zones (210, 220). 9/ A deviceaccording to any one of claims 1 to 8, characterized by the fact that ithas at least one pair of electrodes (310 & 312; 320 & 322) disposedrespectively on either side of the waveguide (110, 410), each of theelectrodes (310 & 312; 320 & 322) being split into two groups ofelectrodes of thickness that is smaller than the gap between the twoplates (100, 400) and respectively adjacent to each plate (100, 400).10/ A device according to claim 9, characterized by the fact that it hasmeans suitable for applying an electrical voltage firstly across twoelectrodes (320 a & 322 a) carried by a first plate (100) and secondlybetween two electrodes (320 b & 322 b) carried by the second plate (400)so as to define electric fields oriented parallel to the plate. 11/ Adevice according to claim 9 or claim 10, characterized by the fact thatit has means suitable for applying an electric voltage firstly between afirst pair of electrodes (312 a & 310 a) carried respectively by the twoplates (100, 400), and secondly between a second pair of electrodes (312b & 310 b) likewise carried respectively by the two plates (100, 400) soas to define electric fields oriented perpendicularly to the plates(100, 400). 12/ A device according to any one of claims 1 to 11,characterized by the fact that it comprises two substrates (100, 400)each possessing an optical waveguide (110, 410) one of the waveguidesdefining an inlet port (112) and an outlet port (114), and the otherwaveguide defining at least one outlet port (414), corresponding to a1×2 configuration. 13/ A device according to any one of claims 1 to 11,characterized by the fact that it comprises two substrates (100, 400)each possessing an optical waveguide (110, 410) each waveguide defininga respective inlet port (112, 412) and a respectively outlet port (114,414) corresponding to a 2×2 configuration. 14/ A device according to anyone of claims 1 to 11, characterized by the fact that it comprises twosubstrates (100, 400) at least one of which possesses a plurality ofoptical waveguides (110, 410) each defining an inlet port (112, 412) andan outlet port (114, 414), corresponding to an N×P configuration. 15/ Adevice according to claim 14, characterized by the fact that the twosubstrates (100, 400) define N(N−1)/2 switches. 16/ A device accordingto claim 14 or claim 15, characterized by the fact that at least some ofthe waveguides (110, 410) possess curved segments. 17/ A deviceaccording to any one of claims 14 to 16, characterized by the fact thatsome of the segments of the waveguides (110, 410) provided on the twosubstrates (100, 400) are placed facing one another, these segmentsbeing separated by pairs of active liquid crystal zones (210, 220) eachassociated with a respective pair of electrodes (310 & 312 and 320 &322). 18/ A device according to any one of claims 1 to 17, characterizedby the fact that it comprises N waveguides (110, 410) provided inalternation on two substrates (100, 400) for waveguides of ranks 1 toN-1, the waveguide of rank i being coupled via i-1 switches to a firstwaveguide of the other substrate and by i switches to a second waveguideof said other substrate. 19/ A device according to any one of claims 1to 11, characterized by the fact that at least one of the substrates(100, 400) carries a plurality of waveguides (110, 410) and that eachwaveguide (110, 410) has various segments (111, 113, 411, 413) that arenot in alignment but that each waveguide (110) provided on a firstsubstrate (100) presents a segment (113) facing each waveguide (410)provided on the second substrate (400). 20/ A device according to claim19, characterized by the fact that each pair of facing segments of twowaveguides (110, 410) defines an assembly with two separate active zones(210, 220) and two orthogonal pairs of electrodes (310 & 312; 320 &322). 21/ A device according to claim 19 or claim 20, characterized bythe fact that the waveguides (110, 410) situated respectively on the twosubstrates (100, 400) extend in directions that are generally mutuallyorthogonal. 22/ A device according to any one of claims 19 to 21,characterized by the fact that each waveguide (110, 410) has a staircaseconfiguration made up of mutually parallel rectilinear segments (111;411) interconnected by segments (113, 413) that are inclined, preferablyat 45°, relative to said parallel segments. 23/ A device according toany one of claims 1 to 22, characterized by the fact that it has meanssuitable for applying electrical voltages across the electrodes (310 &312, 320 & 322) associated with the two active liquid crystal zones(210, 220) sufficient either to reorient the liquid crystal, or to breakthe anchoring of the liquid crystal, so as to form a light switch. 24/ Adevice according to any one of claims 1 to 22, characterized by the factthat it has means suitable for applying a voltage across the electrodes(310 & 312, 320 & 322) associated with the two active liquid crystalzones (210, 220) suitable for controlling the orientation of the liquidcrystal so as to form an optical attenuator.